Street Demon 750 Carb Review

When it comes to carburetors, we find that most guys will fall into one of two camps. There are the hard-core racers/tuners, for whom the bigger it is and the more parts and pieces can be swapped around and fiddled with the better. Then there are the guys who just want one feature: a carb that will bolt onto the engine and work with a minimum of fuss and get the job done. Of course, there are crossovers in these categories of users and in carburetors too. We have met plenty of guys who will buy Holley HP-series race carbs, with replaceable jets and orifices at every conceivable tuning point, bolt them on, and just as soon bolt on something else looking for better performance without busting out a screwdriver. Others will pull a Thermo-Quad from the junkyard and embark on the science project of a lifetime, pin drills and grinder out, massaging every orifice looking to create a magic carb.

The Street Demon, as the name implies, isn’t a carb looking to up-seat a race Holley on the dragstrip. Instead, it is a fuel mixer that takes a common-sense approach to a user-friendly street piece. How so? Start with the basic configuration. The Street Demon is designed to bolt onto a common Holley 4150-style square-bore flange. This makes the carb compatible with everyman’s aftermarket intake manifold and a good number of OEM four-barrel intakes. OK, so it bolts on, that’s a good start. You might notice the carb flange is double-drilled, extending the compatibility to most spread bore Q-Jet/ Thermo-Quad manifolds as well. A single fuel line connection and a full array of necessary vacuum fittings makes the carb compatible with the usual street requirements for vacuum advance and positive crankcase ventilation and even stock EGR. Hooking this carb up to a traditional-style engine will present no unforeseen difficulty. Even most factory four-barrel air cleaners fit without interference, unlike with center-hung bowl Holley carbs.

A Closer Look

Street Demon carbs come in a unique three-barrel throttle-plate configuration, with the rear barrels actually merged into a single-throttle butterfly. Demon calls this the goggle valve secondary, and the arrangement opens considerably more area to secondary airflow. Like a conventional spread-bore carb, the goggle valve really steps up the airflow when the throttle is opened up. The ingenious aspect of the Street Demon is, by using the three-barrel goggle valve secondary, the large secondary airflow is accessible without having to go to a dedicated spread-bore manifold arrangement. The carbs are available in 625 and 750 sizes, with the difference being in the primary bore diameter. You will find 13⁄8-inch primary bores in the smaller version and 11-1⁄16-inch primaries in the large version.

Three major castings comprise the carb’s body, with a separate baseplate, a center float bowl, and an upper air horn casting. All of the major tuning aspects are found in the upper air horn casting, including the replaceable metering rods, and both the primary and secondary jets. The idle mixture screws are conventionally located in the baseplate. The center casting is the float bowl, and it is available in an aerospace composite (phenolic) material, reminiscent of the classic Carter Thermo-Quad carburetor, said to keep fuel temperatures about 20 degrees cooler than an all-metal carb, or the more typical aluminum construction.

Given the basic configuration and shape, as well as the availability of a composite center body, comparisons to the old Carter Thermo-Quad are obligatory. The Street Demon is not a remake of a Carter Thermo-Quad, though it takes some of the TQ’s best features and in many ways improves on the design. Like the TQ, the primary barrels feature triple venturi for a very amplified fuel signal—hastening response and improving efficiency. Like the TQ, the secondary side adds the biggest share of the airflow. Secondary airflow is controlled via an adjustable spring-loaded secondary air door, as pioneered by the Carter AVS and TQ carburetors. Both carbs come with phenolic bowls, optional on the Street Demon.

The benefit of the phenolic bowl is clear, especially in hotter climates, with cooler fuel temperature and a reduced tendency toward fuel percolation or boiling. Here the Demon does away with the leak-prone main wells of the TQ and its bowl-mounted primary jets, moving the primary jetting to stanchions coming down from the upper air horn. The result is a bowl with nowhere to leak. Primary jets and their associated metering rods are unique to the Demon carb, while the secondary jets are common Holley items. Interestingly, the needle and seats, floats, and accelerator pump plunger of the Demon are interchangeable with Thermo-Quad items.

Drive Time

Our look at the Street Demon was more involved than a simple show and tell. Part of our evaluation was a real drive-time evaluation. Our warmed-over engine featured a mild 360 wearing headers, duals, a ported Edelbrock Performer intake, and a TQ carb. The TQ was an 800-cfm unit, nicely tuned and perfectly functional. What we wanted to see was whether the Street Demon lived up to its “ready to run” claim and delivered the street performance we’d expect. The Street Demon bolted in place of our existing carb in minutes, with an Edelbrock carb linkage adaptor allowing for an OEM-style Mopar throttle-linkage/kick-down stud. We swaged a 3⁄8-inch flared fuel fitting to mate with the factory 5⁄16-inch fuel system of our vintage truck and plugged in the required hose connections for the PCV and vacuum advance. Our TQ kick-down linkage required adjustment to meet the square-bore position of the Demon carb, but other than that, the Street Demon was an easy swap for our factory spread-bore carburetor. With a square-bore 4150 flange intake manifold, the installation would have been even easier. With a turn of the key, we had it fired.

Ready to run it was, as the only adjustment required was a twist of the knurled idle adjustment screw to bring us to a 750-rpm idle. The following clandestine WOT speed runs showed that the carb was truly “ready to run.” Off-the-line response easily overcame the 3.55:1 peg leg from a dead stop, bellowing impressive smoke through the neighborhood. Through the gears, the carb pulled clean, leaving us nothing to complain about. After the install, I had a four-hour return drive home and hit the road confident that the Demon would get me back. I have to confess that my intent was to bolt my sweet TQ right back on after testing the Street Demon. The proof is in the driving, and this new carb is here to stay.

Drawing similarities to OEM carburetors, the natural comparison to the Street Demon is the famous Carter Thermo-Quad of the ’70s. One look at the outward appearance of these carbs should be enough to convince even the resolute skeptic that the Demon is an all-new design.Admittedly, some of the hard parts, such as floats, the accelerator pump plunger, and the needle and seat are interchangeable with the production Carter carb, giving the Street Demon ready serviceability with an existing carburetor design.The Street Demon is available in all-aluminum construction or with the heat-insulating phenolic resin bowl. These carbs are available in an upsized configuration, taking the airflow to 750 cfm from its original size of 625 cfm. That level of carb capacity is enough to feed a healthy street-performance engine.With street performance in mind, the Street Demon comes with a very functional electric choke provision. The one-wire hookup does away with the need for a factory-style choke heat well, making it universal for any aftermarket intake manifold.Like the OEM AVS and Thermo-Quad carburetors, the Street Demon takes advantage of a secondary air door and free-flowing venturi-less secondary bores. Fuel is induced in the secondary side via a spray-bar arrangement.The Speed Demon uses an OEM-style progressive secondary linkage. The idle speed is adjusted via a knurled knob, requiring no tools at all for idle speed adjustment.Idle mixture adjustment is via a conventional idle mixture screw, located conveniently at the front of the carb.Primary mixture adjustment is via a two-step metering rod arrangement similar to the AFB/Edelbrock carburetors. The rods are easily accessible via the access plates, and the rate can be tuned by virtue of a simple spring change.The 750-cfm version of the Street Demon features primary throttle plates enlarged to 1-11⁄16-inch from the 13⁄8 size of the original 625-cfm version. Secondary diameter measures 1-3⁄4-inch, plus the added area gained by the goggle valve design.Comparatively, Thermo-Quad carbs came with 1-3⁄8- or 1-1⁄2-inch primary bores and 2-1⁄4-inch secondary plates. The TQ design, however, was limited to spread-bore-style manifolds. We had a small primary TQ on our test engine.The optional phenolic fuel bowl is worthwhile for its cooler running temperature. Note that, unlike the factory TQ, the Street Demon has no jets in the bowl and no glued-on main wells eliminating potential paths for leakage.The secondary air-valve opening rate is a balance between the valve’s spring tension and airflow. Unlike the Thermo-Quad, which is awkward to adjust, the Street Demon is adjustable for air-door tension with a common wrench and screwdriver.You will find virtually all of the tuning points incorporated into the upper air horn casting of the Street Demon, including primary and secondary jets as well as the metering rods. Similarities to the TQ here are the floats and accelerator pump system.Here we can see the primary and secondary metering arrangement. The unique, large, tapered jet at the top works with the metering rods to supply the primary main circuit, while the secondary utilizes a standard Holley jet to meter fuel to the slash-cut secondary discharge nozzles.We bolted the Street Demon 750 to our warmed-over 360 and found excellent street performance right out of the box. We bolted the new carb in place and hit the road for a 250-mile drive with nothing but goodness coming from the big Street Demon carb.